VIBRATIONAL SPECTRA OF HEXAAQUA ... - Science Direct

Joumal of Molecular Structure, 293 (1993) 101-104 Elsevier Science Publishers B.V., Amsterdam

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VIBRATIONAL SPECTRA OF HEXAAQUA COMPLEXES : IV. MULTIPLE BANDS IN THE HOH BENDING REGION OF SOME ALUMS Bojan Soptrajanov Institut

and Vladimir M. Petrugevski

za hemija, PMF, Univerzitet “Kiril i Metodij”, P.O. Box 162, 91001 Skopje,Macedonia

The IR and Raman spectra of a number of sulfate and selenate alums were recorded at room and liquid nitrogen temperature (LNT). At room temperature, the HOHbending vibrations appear as an unsymmetrical, poorly resolved band. remarkable changes in the number of bands When the temperature is lowered, In the LNT spectra of a alums, a progression of and their shape take place. with an almost regular spacing (z 100 cm-i) between the bands could be seen, individual components. In the spectra of the I3 alums, the spectral picture is more complicated and no regularity is readily detectable. In both cases bands region related to the water bending modes appear in a very wide spectral It is possible that an acceptable explanation (from z 2000 to 2 1200 cm-l). for the appearance of multiple bands is based on the assumption of anharmonic coupling of the HOHbending vibration with a low-frequency mode via a mechanism similar to that discussed by Bertie and Falk [ll.

1. INTRODUCTION The alums are a very large and comhaving a mon class of double salts, general formula M’MI’I (R04)z l12HzO. M’ is a univalent cation (such as MII1 Nat , K+, Rb+, Cs+, NH4+etc.), is a tervalent cation (an ion of Al, Ga, In, Fe, Cr etc.) and R is S or conSe. Standardized abbreviations sisting of the chemical symbols for and R and the letter D M’ , MI11 standing for dodecahydrate will be Thus, used throughout the text. the abbreviation CsAlSeD will stand for CsAl(Se04)z l12HzO, AAlSD for NH4Al(S04)2 l12H20 etc. The alums have been very extensively 0022~2860/93/%06.00 Q 1993Ekvier

studied by both crystallographic and spectroscopic methods (see [2-71 and the references given therein). It is known that two types of trigonally coordinated water molecules exist in the structure. The water molecules coordinated to the M3+ ions are strongly hydrogen bonded and those from the Mi(H20)s groups form hydrogen bonds of moderate strength only. Some time ago [8], we observed that the HOHbending bands in the IR and Raman spectra of the alums show multiple structure. Despite the nustumerous published spectroscopic dies, no mention was made (either before or after our communication) of this peculiarity. In fact, either

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102

only the region below 1200 cm-1 was discussed or the HOH bending bands were simply disregarded. The problem with the multiple bands appearing in the HOH/DOD bending region of some crystallohydrates, however, seems to be far from uninteresting. Therefore, in this paper we present our results on the multiple HOH bending bands in the spectra of the alums.

2. EXPERIMENTAL I

The studied compounds were prepared by crystallization from stoichiometric mixtures of aqueous solutions of MIzR04 and M111z(R04)3. The IR spectra were recorded from KBr pellets on a Perkin Elmer 580 IR spectrophotometer. A VLT-2 (RIIC, London) cell was used for the LNT work. The Raman spectra were recorded on a Carry 81 spectrophotometer. The source was a Spectra-Physics Art laser, operating at 514.52 nm.

Fig. the

1. BT

The

HOH

(upper) IB

1400

1600

1800

and

spectra

bending LNT of

region

(lower

in

curve)

CsCrSD

3. RESULTS AND DISCUSSION The room temperature (RT) and LNT IR spectra of CsCrSD are presented in Fig. 1. The broad and poorly resolved HOH bending band at room temperature splits into a number of subbands at LNT. The temperature effect is, as seen, very pronounced. The subbands cover a very wide range of frequencies (1900 - 1300 cm-i in this particular case). Obviously, they can not be interpreted as due to the correlation field splitting components, since no examples of hydrates in which the HOH bending frequency is above 1750 or below 1470 cm-i are known so far.

Fig. IB l-

2.

HOH

bending

bands

in

the

LNT

of some sulfate a alums : 3 - RbAlSD; 2 - KCrSD; 5 - AAlSD 4 - BbCrSD;

spectra KAlSD;

103

1800

Fig. IR

3.

HOH

spectra

1 - RbAlSeD;

1600

1400

bending

bands

of

selenate

some

RbCrSeD;

1800

t

in

the

LNT

alums

:

3 - CsCrSeD;

4 - KAlSeD

Fig.

4.

HOH

IR spectra 1 - CsAlSD;

1400

1600

bending

bands

-

in

the

LNT

of some sulfate 13 alums : 2 - CsGaSD; 3 - CsCrSD;

4 - CsVSD;

5 - RbVSD

Further examples are given in Figs. 24 where the LNT infrared spectra of several sulfate and selenate alums of the a and I3 types are presented. Multiple bands in the 2000 1200 cm-i region could be seen in all these cases. Despite the different selection rules and the excitation mechanisms, multiple bands exist in the lowtemperature Raman spectra as well. As an example, the Raman spectrum of RbAlSeD is presented in Fig. 5.

Fig. 5. The HOH the RT (upper) and

In the case of a alums (Figs. 2 and sequence of 3), an almost regular bands is found, with an average spacing of some 100 cm-l. On the other hand, the spectral picture in the case of the B alums is more involved (cf. Fig. 4).

Second-order transitions, involving librational modes of the water molecules (possibly in Fermi resonance with the HOH bending mode) may, of course, be invoked as an explanation for the origin of these peculiar bands. However, the sequence of

18@0

Raman

1600

spectra

1400

3

bending region in LNT (lower curve) of

RbAlSeD

104

bands found in the a alums would, in that case, be a mere coincidence and this does not seem as an acceptable explanation. Alternatively, these bands could be regarded as a progression, similar to the Frank-Condon progressions which sometimes appear in the electronic spectra. The acceptance of such an explanation would lead to a mechanism similar to that described by Bertie and Falk [l]. These authors, namely, studied the multiple bands in the bending Cl-H***0 (not HOH) region of some ether - HCl complexes and concluded that the bands are due to combinations of the fundamental bending vibration with the low-frequency (50 cm-l) mode assigned to the rocking H-Cl vibration. The observed temperature-induced changes in the case of alums, however, are not in a full agreement with the model since all “hot” transitions should vanish at low temperatures (in the alum spectra the corresponding bands are found below 1600 cm-l). In any case, the temperature effects do suggest that an extensive anharcoupling of the HOH bending manic bands and some low frequency mode(s) since the “pure” HOH bending occurs (expected to appear at 2: 1600 cm-i) should not be temperature sensitive. This frequency is, namely, so high that the excited states at RT could be treated as unpopulated and thus, as mentioned, no remarkable temperature effect is possible. So, the mechanism of Bertie and Falk [II does not seem to be integrally applicable in the present case, but its basic idea (anharmonic coupling with low frequency motions) seems to be operative. Unfortunately, it provides no explanation for the regula-

rity of the sequence spectra

of

found

in

the a alums.

the

Further work, based on quantummechanical model calculations, is needed in order to reveal the true cause for the appearance of multiple HOH bending bands.

ACKNOWLEDGEMENT The financial support by the Ministry for Science of the Republic of Macedonia is sincerely appreciated.

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Bertie

and M.V. Falk, J. Chem., 51 (1973) 1713.

Can.

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S.P. Best, J.K. Beattie and R.S. Armstrong, J, Chem, Sot., Dalton Trans., 1984, 2611.

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and

H.H. Eysel,

Phys. Chem., 92 (1990) 6.

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B. M.

Soptrajanov, V, PetruHevski, Ristova and M. Trpkovska,

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1980.